Due to good resistance to solvents, excellent mechanical strength, electrically insulating properties and size stability, epoxy resin has been widely applied to coating materials, electrically insulating materials, printed circuit laminated boards and electronic packaging materials, construction and building materials, adhesives, and navigation technology. However, epoxy resin has primary defects of poor thermal resistance and burning easily, which may set significant restriction on the uses of epoxy resin. Therefore, with development of electronic technology, it is desired to improve flame retardant properties and thermal resistance of epoxy resin in compliance with increase in the requirements for these properties.
There have been a plurality of techniques available for improving the flame retardant properties of epoxy resin, the most common one of which is to introduce a flame retardant into an epoxy resin compound or a hardener thereof. Generally, a halogen-containing flame retardant is used. Although halogens are effective for retarding flames, they would produce erosive and toxic hydrogen halide gases. According to relevant legal rules and in concern of environmental protection, the halogen-containing flame retardant is getting prohibited in use. Therefore, it is critical to develop a novel flame retardant.
It has been reported that a phosphorus-containing flame retardant has significant advantages of low toxicity, good processing properties, low usage amount, and good compatibility with resin, making the phosphorus-containing flame retardant gradually accepted in wide applications. During a burning process of the phosphorus-containing flame retardant, on the one hand, polymeric materials are urged to undergo a dehydration reaction by which hydrogen of carbohydrate reacts with oxygen of air to form water so as to reduce an ambient temperature and thereby provide a flame retardant effect. On the other hand, phosphoric acid is decomposed under a high temperature, making polymeric compounds carbonized to form a flame retardant coke layer; moreover, phosphoric acid would be further dehydrated and esterized under the high temperature to form glass-like melted polymeric phosphoric acid that covers surfaces of burning substances and serves as a protective layer for preventing oxygen from entering into non-burning internal portions of polymers and for impeding release of volatile decomposed substances, thereby inhibiting proliferation of flames and achieving the flame retardant effect.
Currently used phosphorus-containing substances can be divided into reactive phosphorus-containing compounds with function groups, and generally non-reactive phosphorus-containing compounds. The non-reactive phosphorus-containing compounds have relatively poor thermal resistance and are not suitably applied to epoxy resin compositions required to be highly thermal resistant. The reactive phosphorus-containing compounds bonded to other molecules can thus have relatively higher thermal stability and thereby become a mainstream of usage.
Among available reactive phosphorus-containing compounds, the most commonly used is a linear phosphorus-containing compound; however, due to an —O—P—O— bond on a main chain thereof, this linear phosphorus-containing compound has poorer thermal resistance than a normal halogen-containing or halogen-free epoxy resin composition. In another aspect, phosphorus-containing flame retardant resin compositions, no matter having linear phosphorus-containing compounds or non-reactive phosphorus-containing compounds, are worse in processing properties than bromine-containing epoxy resin compositions in practical applications. Therefore, it is deemed hard to enhance both the flame retardant properties and thermal resistance of the resin compositions.
In accordance with the foregoing problems, it is here to provide a novel phosphorus-containing compound and a preparation method thereof. This phosphorus-containing compound has a different side chain structure from a conventional linear phosphorus-containing compound, which not only can react to form phosphorus-containing epoxy resin by means of various addition or epoxidation reactions but also can directly act as a hardener in an epoxy resin composition. This would effectively improve flame retardant properties and thermal resistance of the resin composition, thereby particularly suitable for epoxy resin compositions required to be highly thermal resistant.